In recent years, drugs that inhibit T-cell checkpoints such as CTL-associated antigen 4 (CTLA-4) and programmed cell death protein 1/programmed cell death ligand 1 (PD-1/PD-L1) have shown remarkable efficacy in several cancers, including melanoma (9–12), non–small cell lung cancer (13–16), renal cancer (17–19), microsatellite instability-high carcinomas of various sites (20), and head and neck cancer (21, 22). Here, PDCD1 is linked to cancer.